if you want easy, then it will be basic. i did something similar, and it was not at all easy... but i think i can explain a simplified version without all the bells and whistles.

in my case, i set up a system that calculated the number of vacation days based on hire date. it also had the ability for HR to add additional "vacation" days based on special circumstances.

in your case, the easy road (which requires more manual entry): create a list that has each employee, and field that has the total number of vacation days (that's the manual entry part... 'cause i suppose you're tracking how that total is calculated somewhere else... like an HR system or excel or something). but anyway, you now have a list with 2 columns: employee, and number of vacation days.

create another list that is a Request list. this needs employee name, date entered, date(s) requested, manager (a more complex system would be able to look up the manager... but again... we're going for "easy" here).

create a workflow on your Request list that triggers on New item. the job of this workflow is to send an email to your Approver (the manager). it copies the fields from the Request list to an Approval list, but with a new column for unhandled/approve/disapprove. this Approval list also has a workflow that triggers when an item is changed... i.e. when the manager selects approve or disapprove.

your "easy" road means the manager will need to go and verify that the employee has remaining vacation days.

when the manager selects approve/disapprove, the workflow on that list will then email the employee with the info, update the total number of vacation days on the Vacation Days Remaining list, and copy the approved items to a Vacation Approved list. you can configure the list to be calendar view.

there are security considerations (i.e. only managers should have access to the Approval list, for example ... and again... the basic approach is that all managers can access this list and there's no restrictions on which employees a manager can approve items for... )

and you'll likely need some other fields or workflow steps...
but you can do this with out of the box custom lists and workflows. just think of things step by step, and copy items to new lists based on the step you're at. don't try to do everything in one list, or with one workflow.

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